Add ninja generator flag for writing library deps.
Add ninja generator flag for writing library deps.

This adds a flag to the ninja generator to specify a file to write library
deps to as generation proceeds. This is so GN can suck the deps out.

BUG=288991
Review=https://codereview.chromium.org/23601020/

old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google